Right from the first few pages of My System, I learned things that I could immediately put to good use. The subject of tempo is just one of those things about which I've never read a satisfactory description, but finally, thanks to Nimzowitsch, I think I understand the use of tempo in the opening. Another issue about which I've been a bit shaky for some time is the use of rooks and open files. Again, Nimzovitsch does an excellent job of clearing up this subject. If you have studied many games by the masters, you've no doubt come across some rook moves that just don't seem to make any sense. A player slides a rook over to a file that isn't even half-open, never mind open, as if there was some purpose, but the reason is completely opaque and the annotator conveniently fails to justify the rook move. Well, Nimzovitsch calls these things "mysterious rook moves," and explains what they are all about. A definite "aha!" moment for me.
Here's another thing that has perplexed me for quite a while. I've often read a phrase along the lines of "provoke a weakness, fix it as a target, eliminate the target." The discussions about this philosophy have been lacking at best, but as Nimzovitsch lays it out in his description of pawn chains and blockading, the issue emerges with extreme clarity. Another "aha!" moment.
It's interesting to note that the author's definition of a pawn chain is somewhat different than definitions I have read before. The reason for this is fairly clear from the text because Nimzovitsch's definition is necessary for his treatment of how to attack pawn structures. He maybe could have chosen a different phrase to describe this pawn structure, but you'll get the hang of it quickly enough.
Probably the thickest common thread throughout the weave of My System is the center of the board: how to occupy it, how to defend it, how to control it, and when to shift your attack away from the center and onto a flank. I found that last bit particularly interesting because, as with much of this book, Nimzovitsch treats a subject about which I am familiar in a way that clarifies my understanding. I ❤ chess though.